Adenosine is a ubiquitous local hormone/neurotransmitter that acts on four known receptors, the adenosine A1, A2A, A2B and A3 receptors. Adenosine generally serves to balance the supply and demand of energy in tissues. For example, in the heart released adenosine slows the heart by an A1 receptor mediated action in the nodes and atria (Belardinelli, L & Isenberg, G Am. J. Physiol. 224, H734-H737), while simultaneously dilating the coronary artery to increase energy (i.e. glucose, fat and oxygen) supply (Knabb et al, Circ. Res. (1983) 53, 33-41). Similarly, during inflammation adenosine serves to inhibit inflammatory activity, while in conditions of excessive nerve activity (e.g. epilepsy) adenosine inhibits nerve firing (Klitgaard et al, Eur J. Pharmacol. (1993) 242, 221-228). This system, or a variant on it, is present in all tissues.
Adenosine itself can be used to diagnose and treat supraventricular tachycardia. Adenosine A1 receptor agonists are known to act as powerful analgesics (Sawynok, J. Eur J Pharmacol. (1998) 347, 1-11; Giffin et al, (2003) 23, 4, 287-292). A2a agonists have recently been shown to give significant pain relief in conditions of increased pain sensitivity (such as neuropathic and inflammatory hyperalgesia) (WO 2004/052377; WO 2004/078183; WO 2004/078184; WO 2005/084653) and are known to have anti-inflammatory activity (see, for example U.S. Pat. No. 5,877,180; WO 99/34804; Linden et al, Expert Opin. Investig. Drugs (2005) 14, 7, 797-806; Sitkovsky et al, TRENDS in Immunology (2005) 26, 6, 299-304; Linden et al, Journal of Immunology (2006) 117, 2765-2769; Cronstein et al (2004) 25, 1, 33-39). In experimental animals, A2A receptor agonists have been shown to be effective against a wide variety of conditions including sepsis (Linden et al, The Journal of Infectious Diseases (2004) 189, 1897-1904), arthritis (Cohen et al, J. Orthop. Res. (2005) 23, 5, 1172-1178; Cohen et al, J. Orthop. Res. (2004) 22, 2, 427-435), and ischaemia/reperfusion injury arising from renal, coronary or cerebral artery occlusion (see, for example Day et al, J. Clin. Invest, (2003) 112, 883-891; Linden et al, Am. J. Physiol. Gastrointest. Liver Physiol. (2004) 286, G285-G293; Linden et al, Am J. Physiol. (1999) 277, F404-F412; Schlack et al, J. Cardiovasc. Pharmacol. (1993) 22, 89-96; Zu et al, J. Cardiovasc. Pharmacol. (2005) 46, 6, 794-802; Linden et al, Am J. Physiol. Heart Circ. Physiol. (2005) 288, 1851-1858; Kennedy et al, Current Opinion in Investigational Drugs (2006) 7, 3, 229-242). The common factor in these conditions is a reduction in the inflammatory response caused by the inhibitory effect of this receptor on most, if not all, inflammatory cells. A2a agonists are also known to promote wound healing (Montesinos, Am. J. Pathol. (2002) 160, 2009-2018).
However, the ubiquitous distribution of adenosine receptors means that administration of adenosine receptor agonists causes adverse side effects. This has generally precluded the development of adenosine-based therapies. Selective A1 receptor agonists cause bradycardia. A2a receptor agonists cause widespread vasodilation with consequent hypotension and tachycardia. The first selective A2A receptor agonist (2-[4-(2-carboxyethyl)phenylethylamino]-5′-N-ethylcarboxamidoadenosine, or CGS21680), was tested in a Phase 2A clinical trial as a potential anti-hypertensive. However, administration of this compound caused a large fall in blood pressure and consequent increase in cardiac output. This has prevented use of CGS21680 as a medicament. Webb et al. (J. Pharmacol Exp Ther (1991) 259, 1203-1212), Casati et al, (J Pharmacol Exp Ther (1995) 275(2):914-919), and Bonnizone et al, (Hypertension. (1995) 25, 564-9) show that selective A2A adenosine receptor agonists cause hypotension and tachycardia. The degree of tachycardia induced is sufficient to preclude their use as medicaments. Alberti et al, (J Cardiovasc Pharmacol. (1997) September; 30(3):320-4) discloses that selective A2A adenosine receptor agonists are potent vasodilators that reduce blood pressure and induce marked increments in heart rate and plasma renin activity. These side effects preclude their use as medicaments.
U.S. Pat. No. 5,877,180 relates to agonists of A2A adenosine receptors which are stated to be effective for the treatment of inflammatory diseases. The preferred agonists, WRC0090 and SHA 211 (WRC0474), are disclosed to be more potent and selective than previously reported adenosine analogs such as CGS21680 and CV1808. Administration of SHA 211 or WRC0090 is considered to reduce the possibility of side effects mediated by the binding of the analogs to other adenosine receptors. However, only in vitro data relating to the activity of SHA 211 is included. There is no demonstration that any of the compounds described could be therapeutically effective in vivo without causing serious side effects. Although side effects mediated by the binding of potent and selective adenosine A2A receptor agonists to other adenosine receptors is expected to be reduced by use of such agonists, the ubiquitous distribution of adenosine receptors means that these compounds would still be expected to activate adenosine A2A receptors in normal tissue and, therefore, cause serious side effects (such as hypotension and reflex tachycardia).
U.S. Pat. No. 3,936,439 discloses use of 2,6-diaminonebularine derivatives as coronary dilating and/or platelet aggregation inhibitory agents for mammals. In vivo data in dogs is included to support the coronary dilating action of N2-Phenyl-2,6-diaminonebularine, N2-Cyclohexyl-2,6-diaminonebularine, N2-(p-methoxyphenyl)-2,6-diaminonebularine, and N2-Ethyl-2,6-diaminonebularine, and in vitro data supports the platelet aggregation inhibitory action of N2-Phenyl-2,6-diaminonebularine, N2-cyclohexyl-2,6-diaminonebularine, 2,6-Diaminonebularine, and N2-Ethyl-2,6-diaminonebularine. FR 2162128 (Takeda Chemical Industries, Ltd) discloses that adenosine derivatives (including 2-alkoxy adenosine derivatives comprising a lower alkyl group of not less than two carbon atoms) have hypotensive and coronary vasodilatory activity. In vivo data in dogs supports the coronary vasodilatory activity of 2-n-pentyloxyadenosine, 2-(β-hydroxyethoxy)-adenosine, and 2-phenoxyadenosine. However, there is no demonstration in U.S. Pat. No. 3,936,439 or FR 2162128 that any of the compounds described could be administered without causing serious side effects.
Ribeiro et al, (Progress in Neurobiology 68 (2003) 377-392) is a review of adenosine receptors in the nervous system. It is stated in the concluding remarks of this article (on page 387, right column, lines 4-10 of section 8) that “as noted a long time ago, activation of adenosine receptors at the periphery is associated with hypotension, bradycardia and hypothermia [ . . . ] These side effects have so far significantly limited the clinical usefulness of adenosine receptor agonists”.
There is, therefore, a need to provide adenosine receptor agonists that can be administered with minimal side effects.
Certain aspects of the invention relate to the treatment of pain. Pain has two components, each involving activation of sensory neurons. The first component is the early or immediate phase when a sensory neuron is stimulated, for instance as the result of heat or pressure on the skin. The second component is the consequence of an increased sensitivity of the sensory mechanisms innervating tissue which has been previously damaged. This second component is referred to as hyperalgesia, and is involved in all forms of chronic pain arising from tissue damage, but not in the early or immediate phase of pain perception.
Thus, hyperalgesia is a condition of heightened pain perception caused by tissue damage. This condition is a natural response of the nervous system apparently designed to encourage protection of the damaged tissue by an injured individual, to give time for tissue repair to occur. There are two known underlying causes of this condition, an increase in sensory neuron activity, and a change in neuronal processing of nociceptive information which occurs in the spinal cord. Hyperalgesia can be debilitating in conditions of chronic inflammation (e.g. rheumatoid arthritis), and when sensory nerve damage has occurred (i.e. neuropathic pain).
Two major classes of analgesics are known: (i) non steroidal anti-inflammatory drugs (NSAIDs) and the related COX-2 inhibitors; and (ii) opiates based on morphine. Analgesics of both classes are effective in controlling normal, immediate or nociceptive pain. However, they are less effective against some types of hyperalgesic pain, such as neuropathic pain. Many medical practitioners are reluctant to prescribe opiates at the high doses required to affect neuropathic pain because of the side effects caused by administration of these compounds (such as restlessness, nausea, and vomiting), and the possibility that patients may become addicted to them. NSAIDs are much less potent than opiates, so even higher doses of these compounds are required. However, this is undesirable because these compounds cause irritation of the gastro-intestinal tract.
There is also a need to provide analgesics, particularly anti-hyperalgesics, which are sufficiently potent to control pain perception in neuropathic and other hyperalgesic syndromes, and which do not have serious side effects or cause patients to become addicted to them.
It has recently become apparent (WO 2004/052377; WO 2004/078183; WO 2004/078184; WO 2005/084653) that some adenosine agonists (e.g. spongosine) are effective analgesics at doses as much as one hundred times lower than would be expected to be required based on the known affinity of this compound for adenosine receptors. At such doses, spongosine and related compounds do not cause the significant side effects associated with adenosine receptor activation. The underlying mechanism behind these observations appears to be that these compounds have increased affinity for adenosine receptors at pH below pH 7.4. It is believed that this property explains the surprising activity of these compounds at low doses. The Applicant has been able to identify certain other compounds that also have increased affinity for adenosine receptors at reduced pH. It is thought that these compounds can be used as medicaments without causing serious side effects. However a significant proportion of these compounds exhibit poor oral bioavailability and short plasma half lives, thus limiting their usefulness as therapeutics.
Spongosine was first isolated from the tropical marine sponge, Cryptotethia crypta in 1945 (Bergmann and Feeney, J. Org. Chem. (1951) 16, 981, Ibid (1956) 21, 226), and was the first methoxypurine found in nature. It is also known as 2-methoxyadenosine, or 9H-purin-6-amine, 9-α-D-arabinofuranosyl-2-methoxy. The first biological activities of spongosine were described by Bartlett et al, (J. Med. Chem. (1981) 24, 947-954). Spongosine (and other compounds) was tested for its skeletal muscle-relaxant, hypothermic, cardiovascular and anti-inflammatory effects in rodents following oral administration (anti-inflammatory activity was assessed by inhibition of carageenan-induced oedema in a rat paw). Spongosine caused 25% inhibition of carageenan-induced inflammation in rats at 20 mg/kg po. However, reductions in mean blood pressure (41%), and in heart rate (25%) were also observed after administration of this compound at this dose.
The affinity of spongosine for the rat adenosine A1 and A2A receptors has been determined. The Kd values obtained (in the rat) were 340 nM for the A1 receptor and 1.4 μM for the A2A receptor, while the EC50 value for stimulation of the rat A2A receptor was shown to be 3 μM (Daly et al, Pharmacol. (1993) 46, 91-100). In the guinea pig, the efficacy of spongosine was tested in the isolated heart preparation and the EC50 values obtained were 10 μM and 0.7 μM for the adenosine A1 and A2A receptors, respectively (Ueeda et al, J Med Chem (1991) 34, 1334-1339). Because of the low potency and poor receptor selectivity of this compound it was largely ignored in favor of more potent and receptor selective adenosine receptor agonists.
The use of nucleoside analogues in the treatment of diseases is often limited by poor oral absorption (Han et al, Pharm. Res. (1998) 15(8), 1154-9). Nucleosides are poorly soluble, polar molecules, and these properties make them poorly permeable to systemic membranes, such as the blood-brain barrier and the cellular membranes that provide access to the drugs' targets (Kling, Modern Drug Discovery (1999) 2(3), 26-36). Thus, oral administration of nucleoside drugs often results in poor or irreproducible in vivo efficacy as a result of a limited or variable concentration of the drug at the site of action. The design and synthesis of new nucleoside analogues therefore remains a very active area of research, with the goal of discovering drugs with optimal oral bioavailability (Dresser et al, Drug Metabolism and Disposition (2000) 28, 9, 1135-40).
Numerous research groups have attempted to solve the problem of poor oral bioavailability of nucleoside drugs by employing a pro-drug of the chosen bioactive species. A pro-drug is a drug which has been chemically modified and may be biologically inactive at its site of action, but which will be degraded or modified by one or more enzymatic or in vivo processes to the bioactive form.
The design of nucleoside pro-drugs has focused on improving oral bioavailabilty by the targeting of nucleoside or peptide transporters, through exploitation of enzymatic processes such adenosine deaminase activation, or by the appending of specific substituents to the sugar moiety of the nucleoside, which aid membrane permeation and are then cleaved in vivo to release the active species.
Various pro-drugs of antivirals have been attempted. Most notably, U.S. Pat. No. 4,957,924 discloses various therapeutic esters of the antiherpetic agent, acyclovir. Valacyclovir, the L-valyl ester of Acyclovir, is an oral prodrug that undergoes rapid and extensive first-pass metabolism to yield Acyclovir and the amino acid L-valine. The bioavailability of Acyclovir from oral Valacyclovir is considerably greater than that achieved after oral Acyclovir administration. Oral administration of Valacyclovir produced a greater increase in urinary excretion of Acyclovir (63%), compared with oral administration of Acyclovir itself (19%) (Perry and Faulds Drugs (1996) 52, 754-72). This increase in oral bioavailability has been attributed to interaction of the L-valyl ester moiety of Valacyclovir with the peptide transporter hPEPT1 (Sawada et al, J. Pharmacol. Exp. Ther. (1999) 291, 2, 705-9; Anand et al, J. Pharmacol. Exp. Ther. (2003) 304, 781). An analogous strategy has been used to increase the oral bioavailability of Zidovudine (AZT) (Han et al, Pharm. Res. (1998) 15(8), 1154-9).
Similarly, WO 01/96353 relates to 3′-prodrugs of 2′-deoxy-β-L-nucleosides for the treatment of hepatitis B virus, that are amino acid esters including valyl and alkyl esters, specifically 3′-L-amino acid ester and 3′,5′-L-diamino acid esters. For example, in cynamalogous monkeys, the 3′,5′-divaline ester pro-drug of 2′-deoxy-β-L-cytidine released 2′-deoxy-β-L-cytidine in vivo with 73% oral bioavailability and a 2.28 h (po) half-life, in comparison to an oral bioavailability of 18% and a half-life of 2.95 h (po) following dosing of 2′-deoxy-β-L-cytidine itself.
In an alternative approach, adenosine deaminase activation of pro-drugs to the active species has been exploited. For example, Viramidine has been shown to act as a pro-drug to the chronic hepatitis C drug, Ribavarin. Viramidine is predominantly converted by adenosine deaminase to Ribavarin in the liver and this liver-targeting property has being exploited to circumvent haemolytic anaemia side effects caused by Ribavarin itself. Thus, after multiple oral dosing of [14C]Ribavarin or [14C]Viramidine to monkey, Viramidine yielded three times the drug level in the liver but only half in red blood cells compared to Ribavarin (Lin et al, Antiviral chemistry & chemotherapy (2003) 14, 145-152; Wu et al, Journal of Antimicrobial Chemotherapy (2003) 52, 543-6).
WO 00/71558 discloses the use of pro-drugs that are esters of N6-oxa, thia, thioxa and azacycloalkyl substituted adenosine derivatives that are selective adenosine A1 receptor agonists. Although an increase in observed in vivo efficacy (fall in heart rate) was observed using this strategy, no data is presented proving this to be a result of any increase in oral bioavailability or effective half-life. Sommadossi et al. (WO 2004/003000) have disclosed 2′ and 3′-pro-drugs of 1′, 2′, 3′ or 4′, SS-D or SS-L, branched nucleosides for treating flaviviridae infections but similarly have not demonstrated that these pro-drugs improve oral bioavailabilty or half-life. Dalpiaz et al (Acta Technologiae et Legis Medicamenti (2002) 13, 49 and Pharm. Res. (2001) 18, 531) have reported stability data of 5′-ester pro-drugs of 6-cyclopentylaminoadenosine (CPA) in whole blood and plasma.